High-pressure liquid chromatography (HPLC) solvent delivery systems are used to source either single-component liquids or mixtures of liquids (both known as "mobile phase") at pressures which can range from substantially atmospheric pressure to pressures on the order of ten thousand pounds per square inch. These pressures are required to force the mobile phase through the fluid passageways of a stationary phase support, where separation of dissolved analytes can occur. The stationary phase support may comprise a packed bed of particles, a membrane or collection of membranes, or an open tube. Often, analytical conditions require the mobile phase composition to change over the course of the analysis (this mode being termed "gradient elution"). In gradient elution, the viscosity of the mobile phase may change and the pressure necessary to maintain the required volumetric flow rate will change accordingly.
Other analytical conditions may require that the mobile phase composition remain fixed over time (this mode being termed "isocratic elution"). However, the fixed composition may result from the mixture of multiple components, and often the relative amounts of the components must be controlled to fractional per cent levels in order to achieve the separations goals.
In liquid chromatography, the choice of an appropriate separation strategy (including hardware, software, and chemistry) results in the separation of an injected sample mixture into its components, which elute from the column in reasonably distinct zones or "bands". As these bands pass through a detector, their presence can be monitored and a detector output (usually in the form of an electrical signal) can be produced. The pattern of analyte concentration within the eluting bands, which can be represented by means of a time-varying electrical signal, gives rise to the nomenclature of a "chromatographic peak". Peaks may be characterized with respect to their "retention time", which is the time at which the center of the band transits the detector, relative to the time of injection (i.e. time-of-injection is equal to zero). In many applications, the retention time of a peak is used to infer the identity of the eluting analyte, based upon related analyses with standards and calibrants. The retention time for a peak is strongly influenced by the mobile phase composition, and by the accumulated volume of mobile phase which has passed over the stationary phase.
The utility of chromatography relies heavily on run-to-run reproducibility, such that standards or calibrants can be analyzed in one set of runs, followed by test samples or unknowns, followed by more standards, in order that confidence can be had in the resulting data. Known pumping systems exhibit some non-ideal characteristics which result in diminished separation performance and diminished run-to-run reproducibility. Among the non-ideal pump characteristics exhibited in known pumping systems are, generally, fluctuations in solvent composition and/or fluctuations in volumetric flow rate.
Volumetric flow fluctuations present in known HPLC pumping systems disadvantageously cause retention time(s) to vary for a given analyte. That is, the amount of time that an analyte is retained in the stationary phase fluctuates undesirably as a function of the undesirable volumetric flow fluctuations. This creates difficulties in inferring the identity of a sample from the retention behavior of the components. Volumetric flow fluctuations can result in fluctuations in solvent composition when the output of multiple pumps are summed to provide a solvent composition.
Fluctuations in solvent composition present in known HPLC pumping systems can disadvantageously result in interactions with the system's analyte detector and produce perturbations which are detected as if they arose from the presence of a sample. In effect, an interference signal is generated. This interference signal is summed with the actual signal attributable to the analyte, producing errors when the quantity of an unknown sample is calculated from the area of the eluting sample peak.
The prior art is replete with techniques and instrument implementations aimed at controlling solvent delivery and minimizing perturbations in the output of delivery systems for analytical instrumentation. Myriad pump configurations are known which deliver fluid at high pressure for use in applications such as liquid chromatography. Known pumps, such as one disclosed in U.S. Pat. No. 4,883,409 ("the '409 patent") incorporate at least one plunger or piston which is reciprocated within a pump chamber into which fluid is introduced. Frequency and stroke length of the plunger reciprocating within the pump chamber is controllable to control the flow rate of fluid output from the pump. However, the assembly for driving the plunger is an elaborate combination of elements that can introduce undesirable motion in the plunger as it is driven, which motion makes it difficult to precisely control delivery system output and results in what is termed "noise" or detectable perturbations in a chromatographic baseline. Much of this noise does not result from random statistical variation in the system, rather much of it is a function of a mechanical "signature" of the pump. Mechanical signature is correlated to mechanically related phenomena such as anomalies in ball and screw drives, gears, and/or other components used in the pump to effect the linear motion that drives the piston(s), or it is related to higher level processes or physical phenomena such as the onset or completion of solvent compression, or the onset of solvent delivery from a cylinder
Typical systems known for delivery of liquids in liquid chromatography applications, such as disclosed in the '409 patent and further in U.S. Pat. No. 5,393,434, implement dual piston pumps having two interconnected pump heads each with a reciprocating plunger. The plungers are driven with a predetermined phase difference in order to minimize output flow variations. Piston stroke length and stroke frequency can be independently adjusted when the pistons are independently, synchronously driven. Precompression can be effected in each pump cylinder in any given pump cycle to compensate for varying fluid compressibilities in an effort to maintain a substantially constant system pressure and output flow rate.
However, in such systems known in the prior art there is no guarantee that from one piston stroke to the next, or from one chromatography run to the next, that significant physical events such as the onset or completion of fluid compression or fluid delivery, occur at precisely known pump mechanical positions. Further, in such systems known in the prior art there is no guarantee that from one piston stroke to the next, or from one chromatography run to the next, that attainment of precisely known mechanical positions occurs at prescribed and repeatable points in successive chromatographic runs.
With known delivery systems, at the termination of a chromatographic run each of the active pumps in a system can reside at a seemingly arbitrary mechanical position or phase. If a new run commences directly from an arbitrary mechanical phase, the mechanical signature of the system will differ, negatively affecting the results of the chromatographic analysis.
This is of particular relevance to the gradient mode of chromatography, where system pressure has to vary dynamically to maintain constant flow while solvent viscosity changes. The hydraulic characteristics of the system, particularly the system hydraulic capacitance, vary as a function of the pump mechanical phase. A variation in the system hydraulic capacitance will change the time-constant of the system hydraulic response. Therefore, the tracking of the delivery pressure profile, and the actual volumetric delivery, will not be precisely repeatable from run to run. Over multiple chromatographic runs, relative motion occurs between a chromatographic peak and features in the surrounding baseline which negatively affects the precision of determining peak areas and retention times, and therefore negatively affects the ability of the chromatographer to quantitate the sample amount and to infer the identity of the sample.